I'm a little late to this thread, and if you're still looking I'm sure you become to realize that there is quite a broad range of tools available.

If you're strictly looking at requirements management (because there are many other pure modeling tools also available), there are, what I would say are the lower end tools ($300-500) like CaseComplete from Serlio or Enterprise Architect from Sparx. Both of these have some basic requirements management functionality (and EA has some plugins that do more).

The main players are IBM Rational with DOORS and RequisitePro, and Borland (Now Micro Focus) with CaliberRM that start at $2K per seat.

And there are now a bunch of new SaaS vendors offering solutions on a monthly basis like Accompa, Gatherspace, PSODA and Clariys.

If you are on twitter, follow a guy called BA Software Monitor www.twitter.com/basoftmon

You could also check out the IAG Consulting site -- they're a reseller for a number of the software vendors and they have some good pages with links to free downloads and demos.